The invention relates to the manufacture of products from sheet material with a double-curved shape. The initially flat sheet material is stretched and drawn over a shaping die. During this process, the sheet material undergoes plastic deformation as a result of the combined action of the stretching and contact with the shaping die.
Usually, a process of this type is used for the production of metal objects, such as the leading edge of a wing, transition pieces for the transition between wing and fuselage, aircraft noses, etc. Shaped products of this type can also be used in the automotive industry.
During the production of double-curved objects, the stretching and forming has to be carried out in a relatively large number of separate steps. The reason for this is that the elongation in, for example, aluminium or steel sheet has to be kept within set limits. If these limits are exceeded, the quality of the product will suffer, for example as a result of the appearance of surface lines caused by the crystal structure of the sheet material being affected. Particularly in the case of products which are used in the bright, polished state, this leads to an unacceptable appearance.
In practice, therefore, the sheet material undergoes plastic deformation in small steps. After each shaping step the intermediate product obtained undergoes stress-relief annealing. After cooling, the intermediate product then has to be clamped in place again, be shaped one step further, released from its clamps again, annealed, cooled, etc.
A process of this type is laborious and labour-intensive. Moreover, it should be considered that a lubricant has to be applied to the shaping die before each shaping step, in order to counteract corrosion during shaping. The film of lubricant has to be removed before the intermediate product can be annealed.
Depending on the curvature of the end product and the maximum acceptable elongation in the material, the shaping process may take up several days, for example 3 to 4 days.
Therefore, if the number of steps in the shaping process could be reduced, it would be possible to make considerable savings in terms of production time and costs. A precondition is that the quality of the end product must not suffer when using a higher production rate.
The first object of the invention is to provide an improved method of this type. This object is achieved by means of a method for forming a double-curved object by means of stretching a sheet, comprising the steps of:
providing a shaping die with a shaped die surface,
positioning the sheet in such a manner that it extends along the shaped die surface of the shaping die,
clamping two opposite edges of the sheet into in each case one clamping head which extends along the respective edge,
during a working stroke, moving the sheet with clamping heads and the shaping die towards one another, in such a manner that the sheet comes into contact with a part of the die surface which projects a relatively long way, and, as a result of continued movement of shaping die and clamping jaws, comes into contact with a part of the die surface which in relative terms does not project such a long way,
influencing at least one clamping of the sheet in such a manner that a sheet edge section which is situated next to a part of the die surface which in relative terms does not project such a long way executes a greater advancing movement than a sheet-edge section which is adjacent to a part of the die surface which projects a relatively long way.
In the method according to the invention, the differences in elongation as arise in the sheet material remain limited on account of the use of one or more shaping pieces. These shaping pieces ensure that the sheet material is able to follow even the lower parts of the shaping die during stretching. This prevents those parts of the sheet material which bear against the higher parts of the shaping die from rapidly acquiring excessive elongation. On account of the use of the shaping pieces, therefore, the stretching process can be continued further for each deformation step without locally excessive elongation occurring and without the sheet material being damaged.
In this method, clamping heads which are known per se and comprise a pair of clamping jaws can be used. These clamping jaws each bear against one surface of the sheet. When carrying out the method, a flat sheet is clamped between the pairs of clamping jaws, and as a result of the contact between shaping die and sheet the sheet is bent around the clamping jaws which are situated on that surface of the sheet which is remote from the shaping die. Now, according to the invention, a shaping piece is attached to the said clamping jaw, or a shaped clamping jaw is used.
The use of a shaped clamping jaw or of a separate shaping piece only involves a minor intervention in the machine, which can be carried out without major problems.
The invention also relates to a device for carrying out the method as described above, comprising a frame, two elongate clamping heads which are arranged parallel to one another on the frame, a support with an exchangeable shaping die, on either side of which support there are the clamping heads, and drive means for moving the clamping heads to and fro transversely with respect to their longitudinal direction, along the shaping die in the direction of a working stroke, during which the sheet is stretched and formed, and a return stroke. According to the invention, at least one of the clamping heads is provided with a single-curved or double-curved shaping piece.
The shaping piece may be integrated in the clamping head or may be a separate component which is attached to the clamping head. Each clamping head comprises a pair of clamping jaws, at least one of the clamping jaws, which is situated at the rear in the direction of a working stroke, being provided with a shaping piece.
The shape of the shaping pieces is adapted to that of the shaping die. Since the shaping die comprises a part which in relative terms does not project such a long way in the opposite direction to the direction of the working stroke, at least one of the clamping heads must bear a shaping piece which, approximately at a position which is adjacent to the said part of the shaping die which in relative terms does not project such a long way, comprises a part which does project a relatively long way.
Although the above text speaks of a metal as the sheet material, for example aluminium or steel, the invention can also be used for sheet material made from plastics.